1. Field of the Invention
This invention relates generally to an extreme ultraviolet (EUV) radiation source and, more particularly, to a laser-plasma EUV radiation source where the target area for the laser beam and the target stream are far enough from the source nozzle to provide an isolated plasma for improving the conversion of laser power to EUV radiation.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current trend for photolithography light sources is to develop a system that generates light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13-14 nm).
Various devices are known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically xenon, as a laser plasma target material. Other gases, such as argon and krypton, and combinations of gases, are also known for the laser target material. In the known EUV radiation sources based on laser produced plasmas (LPP), the gas is typically cryogenically cooled to a liquid state, and then forced through an orifice or other nozzle opening into a vacuum process chamber as a continuous liquid stream or filament. The liquid target material rapidly evaporates and freezes in the vacuum environment to become a frozen target stream. Cryogenically cooled target materials, which are gases at room temperature, are desirable because they do not condense on the source optics, and because they produce minimal by-products that have to be evacuated by the process chamber. In some designs, the nozzle is agitated so that the target material emitted from the nozzle forms a stream of liquid droplets having a certain diameter (30-100 μm) and a predetermined droplet spacing.
The target stream is irradiated by high-power laser beam pulses, typically from an Nd:YAG laser, that heat the target material to produce a high temperature plasma which emits the EUV radiation. The pulse frequency of the laser is application specific and depends on a variety of factors. The laser beam pulses must have a certain intensity at the target area in order to provide enough heat to generate the plasma. Typical pulse durations are 5-30 ns, and a typical pulse intensity is in the range of 5×1010-5×1012 W/cm2.
FIG. 1 is a plan view of an EUV radiation source 10 of the type discussed above including a nozzle 12 having a target material storage chamber 14 that stores a suitable target material, such as xenon, under pressure. A heat exchanger or condenser is provided in the chamber 14 that cryogenically cools the target material to a liquid state. The liquid target material is forced through a narrowed throat portion or capillary tube 16 of the nozzle 12 to be emitted under pressure as a filament or stream 18 into a vacuum process chamber 26 towards a target area 20. The liquid target material will evaporate and quickly freeze in the vacuum environment to form a solid filament of the target material as it propagates towards the target area 20. The vacuum environment in combination with the vapor pressure of the target material will cause the frozen target material to eventually break up into frozen target fragments, depending on the distance that the stream 18 travels and other factors.
A laser beam 22 from a laser source 24 is directed towards the target area 20 in the process chamber 26 to vaporize the target material filament. The heat from the laser beam 22 causes the target material to generate a plasma 30 that radiates EUV radiation 32. The EUV radiation 32 is collected by collector optics 34 and is directed to the circuit (not shown) being patterned, or other system using the EUV radiation 32. The collector optics 34 can have any shape suitable for the purposes of collecting and directing the radiation 32, such as an elliptical dish. In this design, the laser beam 22 propagates through an opening 36 in the collector optics 34, as shown. Other designs can employ other configurations.
In an alternate design, the throat portion 16 can be vibrated by a suitable device, such as a piezoelectric vibrator, to cause the liquid target material being emitted therefrom to form a stream of droplets. The frequency of the agitation and the stream velocity determines the size and spacing of the droplets. If the target stream 18 is a series of droplets, the laser beam 22 may be pulsed to impinge every droplet, or every certain number of droplets.
As discussed above, the low temperature of the liquid target material and the low vapor pressure within the process chamber cause the target material to quickly begin freezing as it exits the nozzle exit orifice. This quick freezing tends to create an ice build-up on the outer surface of the exit orifice of the nozzle. The ice build-up interacts with the stream, causing stream instabilities, which affects the ability of the target filament to reach the target area intact and with high positional precision.
Also, filament spatial instabilities may occur as a result of freezing of the target material before radial variations in fluid velocity within the filament have relaxed, thereby causing stress-induced cracking of the frozen target filament. In other words, when the liquid target material is emitted as a liquid stream from the exit orifice, the speed of the fluid at the center of the stream is greater than the speed of the fluid at the outside of the stream. These speed variations will tend to equalize as the stream propagates. However, because the stream quickly freezes in the vacuum environment, stresses are induced within the frozen filament as a result of the velocity gradient.
The evaporating target stream 18 creates a certain steady-state pressure gradient at its location in the vacuum chamber 26. The pressure within the vacuum chamber 26 decreases the farther away from the target stream 18. Electrical discharge arcs are emitted from the plasma 30 to the conductive portions of the nozzle 12 if the gas pressure is high enough to support electrical breakdown. These arcs can travel relatively large distances and will damage the nozzle throat 16, resulting in degradation of the quality of the stream 18. If the local pressure surrounding the stream is low enough, then the electrical discharge arcs cannot be supported. Additionally, fast atoms from the plasma 30 and solid pieces of excess, unvaporized target material can impact the nozzle 12.
The electrical discharge arcs from the plasma 30 cause the nozzle material to melt or vaporize, creating nozzle damage and excess debris in the chamber. Also, the fast atoms and excess target material erode the nozzle 12. This debris also causes damage to the optical elements and other components of the source resulting in increased process costs.
It is desirable that an EUV radiation source has a good conversion efficiency. Conversion efficiency is a measure of the laser beam energy that is converted into collectable EUV radiation, i.e., watts of EUV radiation divided by watts of laser power. Xenon vapor, or other target gas vapor, emitted into the process chamber 26 as the target stream 18 freezes absorbs the EUV radiation 32 directly effecting the source conversion efficiency. For example, if the nozzle exit orifice is only a few millimeters away from the target region 20, about 30% of the EUV radiation will be absorbed. The process chamber 26 is maintained at an average pressure of a few militorr, or less, to minimize the target material vapor within the chamber, and thus, the EUV absorption losses to the target material vapor. When the target stream completely freezes, vapor no longer is emitted therefrom. Therefore, most of the EUV absorbing vapor is close to the nozzle exit orifice.
It would be desirable to move the target area 20 far enough away from the nozzle 12 so that the nozzle 12, and other source components, are not damaged by arcing and fast ions from the plasma 30. Further, by moving the target area 20 far enough away from the nozzle 12, the generated EUV radiation is not significantly absorbed by the target vapor. This provides a cost benefit because less powerful lasers would be required for the same amount of EUV radiation output, and lower vacuum pressures would be necessary. Stream instabilities need to be addressed so that the target stream accurately hits the target area 20.